Jet fuel manufacture

ABSTRACT

JET FUELS WHICH HAVE BOTH A HIGH HEAT CONTENT AND AN ACCEPTABLE LOW TEMPERATURE VISCOSITY ARE PREPARED BY THE HYDROALKYLATION OF MONONUCLEAR AROMATIC HYDROCARBON FRACTIONS. BENZENE, TOLUENE, AND UNDEX EXTRACT, AND MIXTURES THEREOF, ARE THE PREFERRED STARTING MATERIALS. THESE ARE HYDROALKYLATED BY REACTION AT ABOUT 30*-250*C. UN DER SUPER-ATMOSPHERIC PRESSURE OF ABOUT 20-70 ATMOSPHERES OF HYDROGEN, THUS PRODUCING HYDROALKYLATES CONTAINING CYCLOALKYL-SUBSTITUTED BENZENES, METHYLCYCLOHEXYLSUBSTITUTE TOLUENS AND THE LIKE. THESE ARE FURTHER REDUCED TO THE CORRESPONDING CYCLOALKYL-SUBSTITUTED CYCLOHEXANES, TOGETHER WITH METHYLCYCLOHEXYL METHYLCYCLOHEXANES AND METHYLCYCLOHEXANE WHEN TOLUENE IS USED, AND THE REDUCTION PRODUCTS ARE ADJUSTED TO A FINAL DESIRED HEAT CONTENT AND LOW TEMPERATURE VISCOSITY BY STRIPPING OFF OR ADDING METHYLCYCLOHEXANE. THE PREFERRED CATALYST IS FINELY DIVIDED NICKEL ON A SUPPORT CONTAINING BOTH A ZEOLITE AND A SILICA-ALUMINA CRACKING CATALYST.

United States Patent 3,791,958 JET FUEL MANUFACTURE Kenneth L. Dille, Wappingers Falls, George S. Sarnes, Fishkill, and Alfred Arkell, Wappingers Falls, N.Y., assignors to Texaco Inc., New York, NY.

No Drawing. Original application Oct. 13, 1971, Scr. No. 80,481. Divided and this application Nov. 6, 1972, Ser. No. 303,732

Int. Cl. Cl1/04 U.S. Cl. 208- 3 Claims ABSTRACT OF THE DISCLOSURE Jet fuels which have both a high heat content and an acceptable low temperature viscosity are prepared by the hydroalkylation of mononuclear aromatic hydrocarbon fractions. Benzene, toluene, and Udex extract, and mixtures thereof, are the preferred starting materials. These are hydroalkylated by reaction at about 30250 C. under super-atmospheric pressures of about -70 atmospheres of hydrogen, thus producing hydroalkylates containing cycloalkyl-substituted benzenes, methylcyclohexylsubstituted toluenes and the like. These are further reduced to the corresponding cycloalkyl-substituted cyclohexanes, together with methylcyclohexyl methylcyclohexanes and methylcyclohexane when toluene is used, and the reduction products are adjusted to a final desired heat content and low temperature viscosity by stripping off or adding methylcyclohexane. The preferred catalyst is finely divided nickel on a support containing both a zeolite and a silica-alumina cracking catalyst.

This is a division, of application Ser. No. 80,481, filed Oct. 13, 1971.

This invention relates to a method for the production of jet fuels of high heat content from mononuclear aromatic hydrocarbons such as benzene, alkylbenzenes such as toluene, ethylbenzene and the like, and dialkylbenzenes such as the various xylenes, including mixtures thereof such as those obtained in Udex extracts.

A principal object of the invention is to convert these aromatic hydrocarbons into jet fuels by hydroalkylation processes. A further object is the production of jet fuels, having both a high heat content and an acceptable low temperature viscosity, by reducing cycloalkyl-substitnted mononuclear aromatic hydrocarbons to a mixture of cycloalkyl-substituted cyclohexanes.

A still further object of the invention is the provision of new jet fuels wherein the principal heat-producing ingredients are cyclohexyl-substituted benzenes, methylcyclohexyl-substituted toluenes and other mononuclear aromatic hydrocarbon derivatives and their reduction products such as dicyclohexyl-substituted cyclohexanes, polycyclohexyl cyclohexanes and mixtures thereof together with sufiicient methylcyclohexane to impart a desired low temperature viscosity.

Additional objects of the invention will become apparent from the following description of the principles thereof when taken with the attached examples and the appended claims.

It is well known that the turbojet engine operates by the combustion of fuel between an air compressor and a turbine, thereby increasing the volume and temperature of the air so that the work of compression can be taken out of the gases by the turbine with enough energy remaining to impart the high final kinetic energy and speed to the jet that is needed for propulsion. For this purpose a fuel having a high heat content, as measured by its heat of total combustion, is needed. In addition, a fuel having an acceptable low temperature viscosity is needed because Patented Feb. 12, 1974 of the low temperatures which may be encountered during storage and high altitude use.

We have found that jet fuels having both of these properties can be obtained by processes based on the hydroalkylation of mononuclear aromatic hydrocarbons such as benzene, toluene, higher alkylbenzenes, xylenes and the like, including particularly mixtures of these hydrocarbons such as are obtained by extracting aromatic petroleum fractions with suitable solvents. In the preferred practice of our invention these hydrocarbons or hydrocarbon mixtures are hydroalkylated, thereby producing mixtures containing cycloalkyl-substituted mononuclear aromatic hydrocarbons, followed by partial or complete reduction to the corresponding cycloalkyl-substituted cyclohexanes or alkylcyclohexanes. We have found that jet fuels having varying but controlled ratios of heat content to low-temperature viscosity can be obtained by fractionating the reduction products so obtained, or by stripping volatile ingredients such as methylcyclohexane therefrom.

The production of cycloalkyl aromatic compounds by hydroalkylation is a known procedure. It is carried out by reacting mononuclear aromatic hydrocarbons such as benzene, toluene and mixtures thereof with hydrogen in the presence of a mixed catalyst which contains both an alkylation component and a hydrogenation component. Typical catalysts and reaction conditions are described in US. Pats. Nos. 3,274,276 and 3,317,611, and any of the catalysts and reaction conditions described in these patents may be used in practicing our present invention. We have found, however, that good results are obtainable by carrying out the hydroalkylation in the presence of a finely divided nickel catalyst supported on a substantially alkali metal-free mixture of a zeolite, from which substantially all of the alkali metal has been removed, together with a silica-alumina cracking catalyst of the type now employed for the vapor phase cracking of petroleum hydrocarbons into gasoline. This catalyst has the advantage that it can also function as a hydrogenation catalyst in the second stage of our process wherein the hydroalkylate is reduced by further reaction with hydrogen. Alternately, a more efficient hydrogenation catalyst, such as Ni on alumina, may be used during second stage reduction.

In the preferred practice of our invention the hydroalkylation is preferably carried out at temperatures below about 450 F. and preferably within the range of about to 400 F. and pressures of hydrogen of about 20-70 atmospheres. The subsequent reduction of the hydroalkylate may be obtained by continuing the reaction with the same or other hydrogenation catalyst at about 300 to 1,000 psi. and temperatures below 450 F. until the uptake of hydrogen is substantially complete. As a practical matter pressures of hydrogen of about 450 to 550 psi. are preferred in order to lessen the reaction time. The resulting reaction mixture is then preferably distilled at atmospheric or sub-atmospheric pressures to strip 01f unreacted or partially reacted starting materials and byproducts of relatively low heat content.

We have found, as a result of the tests shown in the following specific examples, that the heat of combust1on on both a weight and a volume basis is increased by increasing the number of cyclohexyl substituents on phenyl and also on tolyl and other alkylphenyl groups. In contrast to this, the heat of combustion on a volume basis is increased with an increase in the number of cyclohexyl groups attached to cyclohexane and alkyl cyclohexanes, but on a weight basis the heat of combustion is decreased. In both cases, however, the poly-substituted compounds have relatively high heat of combustion values. These discoveries are important in the production of jet fuels for specific end uses, such as for military planes on the one hand and commercial planes on the other hand. In military jet fuels the heat content is preferably based on the volume of the fuel, because most military applications are volume limited. In commercial jet airplanes the fuel value is evaluated on a weight basis, since space is not so important, and therefore a high heat of combustion on a weight basis is preferred. It is an important advantage that the process of our invention can be adapted to produce jet fuels of either type from relatively inexpensive starting materials such as benzene, toluene, and alkylbenzene mixtures obtained by the solvent extraction of aromatic hydrocarbon fractions. A particularly useful source of such mixtures is Udex extract, obtained by the selective extraction of aromatics from various feed stocks by absorption in aqueous diethylene glycol solutions, and the use of such extracts as starting materials constitutes a particular specific feature of the invention.

In its most practical aspect, therefore, the preferred practice of our invention consists in treating toluene or toluene-containing mixtures such as Udex extract to a 3-stage process. In the first stage the toluene is hydroalkylated as described above to a product containing methylcycloalkyl-substituted toluenes, which are usually mixed with unconverted toluenes since the hydroalkylation is seldom carried to completion. In the second stage this product mixture is reduced by hyrdogenation to form the corresponding methylcycloalkyl-substituted methylcyclo hexanes along with methylcyclohexane from the toluene reduction. In the third stage a part or all of the methylcyclohexane is removed by distillation in order to adjust the jet fuel product to the desired final viscosity and B.t.u. content. Alternately, most of the unconverted toluene from the first stage can be stripped off for recycle to the hydroalkylation unit. Using this procedure there is no excess methylcyclohexane produced during second stage reduction. Jet fuel blends with energy contents in the 136,000 B.t.u. per gallon range are obtained by either of these procedures, and viscosities as low as 50-75 poises at minus 65 C. are obtainable by the presence of about 20% to about 50% of methylcyclohexane in the product. Such blends may contain dicyclohexylbenzene, tricyclohexylbenzene, dicyclohexy cyclohexane or higher polycyclohexyl cyclohexanes, but usually mixtures of two or more of these compounds are present in ratios that will vary with the extent of hydroalkylation and of reduction used in the process.

The invention will be further described and illustrated by the following examples. The catalyst used in these examples consisted of about 6% by weight of finely divided nickel distended on about 94% of a hydrocarbon-cracking catalyst support which was a substantially alkali metalfree mixture of about 22% by weight of a modified zeolite Y, about 58% of silica and about 20% of alumina.

The modified zeolite portion of the cracking component has uniform pore openings of from 6-15 angstrom units, has a silica-alumina ratio of at least 2.5, e.g. 3-10, and has a reduced alkali metal content. The modified zeolite is prepared by subjecting synthetic zeolite Y to ion exchange by contacting the zeolite several times with fresh solutions of an ammonium compound at temperatures ranging between about 100 and 250 F. until it appears that the ion exchange is substantially complete. The ion exchanged zeolite is then washed to remove solubilized alkali metal and dried at a temperature sufiiciently high to drive off ammonia. This converts the zeolite Y to the hydrogen form and reduces the alkali metal content to about 24 weight percent. The ion exchanged zeolite is then calcined at a temperature of about 1000 F. for several hours. After cooling, the ion-exchanged calcined zeolite is subjected to additional ion exchange by contact several times with fresh solutions of an ammonium compound and again washed and dried. This treatment results in a further reduction in the alkali metal content of the zeolite to less than 1% and usually to about 0.5%. It would appear that after the first calcination, it is possible to engage in further ion exchange with the removal of additional alkali-metal ions not removable in the initial ion exchange. Calcination at e.g. 1200 F. may take place here or it may be postponed until after the incorporation of the amorphous inorganic oxide and impregnation with the hydrogenating component at which time the composite should be calcined. Whether calcination is postponed or repeated, the final calcination temperature should not exceed 1200 F.

The silica-alumina portion of the support is a composite of alkali metal-free silica and alumina of the wellknown type used in petroleum cracking and described, for example, in US. Pat. No. 2,469,314. It is frequently prepared by acidifying an aqueous sodium silicate solution with aqueous 25% sulfuric acid, washing the resulting hydrated silica free from alkali metal salts, suspending it in aluminum sulfate solution, precipitating with ammonia, filtering and washing. The filter cake is preferably mixed with a sufficient quantity of the modified zeolite to obtain 22% by weight of zeolite after drying and the mixture is dried and calcined. It is then impregnated with 6% of nickel by adding nickel nitrate solution, drying and heating in the presence of hydrogen.

EXAMPLE 1 (a) Toluene was mixed with about 5% of its weight of the above-described catalyst and hydroalkylated at 375 F. and about 500 p.s.i.g. hydrogen pressure. After 1.5 hours a 31% conversion was obtained with 79% of the hydroalkylate being mixed with isomeric methylcyclohexyltoluenes.

(b) In a second run, 4.5 grams of the catalyst were suspended in 47 grams of toluene in a 100 cc. rocking autoclave and hydroalkylation was continued under the same reaction conditions for 5 hours at which point the toluene conversion was over 90%.

The reaction mixture was fractionated by distillation at 1 torr; i.e. at 1 millimeter of mercury pressure. A total of 17.3 grams of methylcyclohexane and unconverted toluene was first removed. A second cut, taken at 48- 50 C., consisted of 11.9 grams of mixed methylcyclohexyltoluenes. A third cut weighing 2.1 grams distilled over at 120-l27 C. and contained disubstituted compounds as the principal ingredients.

(c) Following these preliminary investigations a larger scale run was made with 460 grams of toluene in a 1- liter rocking autoclave at 370-380 F. and 500 p.s.i.g. pressure using 22.5 grams of the same catalyst. This run was carried out to high conversion and the product was distilled under the conditions and with the results shown in the following table.

TABLE I Distillation Temp., Cut t.,

Press. C. No. grams Products .Atm 1 199 Toluene and traces of MCHa. 20 torr.- 121 22 12!; Mixed MCHTs. 8 torr-.. 1 106408 4, 5 M383 t; and higher substrtu Residue 22 p 1 MCHa=Methylcyc1ohexane; MCHTs=Methylcyclohexyltoluenes.

EXAMPLE 2 Using the same catalyst, reaction conditions and rocking autoclave as in Example 2, 390 grams of benzene was hydroalkylated and reduced to give the following main distillation fractions.

TABLE II Temp. Pressure Out C.) (torr) Products 1 78 Atm. Cyclohexane.

2.. 83 0.4 Dicyclohexane. 4--.--- 123 0.5 Dicyclohexylcyclohexane. Residu Polycyclohexylcyclohexanes.

EXAMPLE 4 Udex extracts are principally mixtures of mononuclear aromatic hydrocarbons obtained by selective ex- EXAMPLE 5 Production of jet fuels We have found that jet fuels which have both a high heat content and an acceptable low temperature viscosity can be prepared by the hydroalkylation and reduction procedures illustrated in Examples 14. By varying the composition of the mononuclear aromatic hydrocarbons and their mixtures from benzene to alkylated benzenes, and by controlling the degree of cycloalkylation and subsequent, hydrogenation of the remaining aromatic ring, it is possible to control the aromatic to naphthenic ratio.

This control allows the synthesis of fuels having properties which adapt them for specific end uses. Thus, for example, jet fuels which have higher aromatic ratios are better suited for military use due to their higher density while lower density fuels for commercial aircraft are obtained by hydrogenation of the aromatic ring which may be followed by light end stripping if desired.

The hydroalkylation products of benzene, toluene and Udex extract and their reduction products were evaluated as jet fuel ingredients, in comparison with methylcyclohexane and i-propyldicyclohexyl, by determining their heats of combustion and their viscosities at minus 65 F. The results with benzene hydroalkylation products are shown in the following table.

TABLE IV 65 F. B.i;.u l Density B.t.u./ viscosity Test number Compound, mixture or blend (g./cc.) gal. (poise) i-Propyldicyclohexyl 18, 430 136, 544 Solid Methylcy ne 18, 569 7688 1... Cyelohexylbenzene (CHB) 17,504 .9473

2-..- Di-CHBs 17,736 9719 3 Di-CHBS plus Wt. percent MCHa 17, 905 9217 187, 74 163 4 Di-OHBS plus wt. percent MCHa 17, 929 9189 137, 500 58 5. TI'i-CHB 17,844 9848 146, 340 6. Dicyclohexane- 18, 478 8838 136, 000 Solid 7. Dicyclohexylcy nn 18, 312 9277 141, 500 8- Dlcyclohexylcyclohexane plus 22 wt. percent MCHa 18, 299 8918 136, 200 163 9 Polyeyclohexylcyolnhemna 18,218 9629 146, 090 10 Polyeyclohexylcyclohexane plus 40 wt. percent MCHa 18, 238 8788 133, 770 163 11 Polycyclohexyleyclohexane plus wt. percent MCHa ,328 8570 131, 100 5 8 1 Methyleyclohexane (MCHa). fl Cyclohexylbenzenes (OHB's).

traction of aromatic-containing feed stocks with aqueous diethylene glycol solution. A typical extract may contain 15.3 volume percent of benzene, 40.4 percent of xylenes and ethyl benzene and 42.5 percent of toluene. They are a very attractive source of raw material for conversion into jet fuels because of their abundance and relatively low cost.

A sample of Udex extract was used which was free from butanes and contained 90-95% aromatics, mostly toluene and higher alkylated benzenes. Gas chromatographic analysis showed that it contained at least six different compounds and, after hydroalkylation, the product mixture contained at least ten components in the monosubstitution region.

The sample was mixed with 5.3% of its weight of catalyst and hydroalkylated at 370380 F. using 500 p.s.i.g. of hydrogen pressure. The rate of hydrogen uptake was slightly less than that of pure toluene but was greater than for the xylenes of Example 3. Distillation gave the following results.

The dicyclohexylbenzenes reported in this table were made by hydroalkylation of benzene mixed with 5% of its weight of the catalyst previously described. Distillation of the hydroalkylate gave dicyclohexylbenzenes as a fraction boiling within the range of 150-165 C. at 1 torr pressure. The tricyclohexylbenzenes used were obtained from the same materials in a rocking autoclave run, the desired fraction (cut 5; boiling range 158-185 C. at 0.5 torr) containing only a trace of dicyclohexylbenzenes. The dicyclohexylcyclohexane and polycyclohexylcyclohexanes were the products of Table II of Example 3. Runs 7-11 show that jet fuels having both high heat contents and acceptable viscosities at low temperatures can be obtained by blending a mixture of polycyclohexylcyclohexanes with about 20% to about 50% of their weight of methylcyclohexane.

Table IV shows that increasing the number of cyclohexyl groups on benzene (compare 1, 2 and 5) increases the heat of combustion on both a weight and volume basis. In contrast to this, increasing the number of cyclohexyl groups on cyclohexane (compare 6, 7 and 9) causes the heat of combustion on a weight basis to decrease and on a volume basis to increase. In both cases, the trisubstituted compounds have relatively high heat of combustion values of over 146,000 B.t.us/gal., but the dicyclohexylbenzenes are better suited for military purposes because of their higher density. Test Nos. 3 and 4 show that their low temperature viscosities can be brought into the desired range by blending them with minor proportions of methylcyclohexane.

7 Table V shows the results with products of Examples 1, 2 and 4 with isopropyldicyclohexyl as a basis of comparison.

be obtained in practicing the process of our present invention.

TABLE V 65 F. B.t.u./ Density B.t.u.] viscosity Test number Compound or mixture Source lb. (g./cc.) gal. (poise) i-Propyldicylcohexyl 1 Methylcycloehcxyltoluenes Ex. 1(a), Cut 2 Highler subst. produets Ex. 1(a), Cut 4,5

Ex. 1(0) residue.

Ex. 2, Cut 2 Ex. 2, rosidum-.. Ex. 4, Cut 3 plus r Although the heat contents of the blends shown in Table IV have been decreased, compared to the parent materials, they are still in the range of 131,000 B.t.u./gal. to 137,500 B.t.u./ gal. A comparison of the most promising blends with materials being considered by the Air Force is shown in the following summary:

Note that the tricyclohexanes have a higher heat content (141,500 B.t.u./gal.) than the structural similar and commercially available isopropyldicyclohexyl (136,500 B.t.u./gal). Tricyclohexanes blended with 22 wt. percent methylcyclohexane have a heat content almost the same as isopropyldicyclohexyl and the blend has distinct viscosity and cost advantages over the latter compound.

The foregoing tables show that the hydroalkylation products of mononuclear aromatic hydrocarbons have heats of combustion in the 135,000 to 146,000 B.t.u./ gal. range, but their flow properties at minus 65 F. require improvement if they are to be used for military purposes. According to Air Force specifications, the desired viscosity for a jet fuel is 5 poise at 65 F. However, it is evident that higher viscosities would be acceptable if the fuel had an unusually high heat value or very low cost. Shelldyne, for example, has a viscosity of 170 poise at -65 F, high heat value of 150,000 B.t.u./gal., and an unrealistically high cost of $42.00/ga1. Using the 170 poise value as a guide, two polymer standards were obtained which had viscosities of 163 and 58 poise at room temperature. The Blends shown in Table IV were made in order to meet or improve on the Shelldyne viscosity without too great a loss in heat content. Methyl cyclohexane (MCH) was used as the diluent both because the Air Force has expressed an interest in MCH from the standpoint of cost, availability, and heat-sink capacity and because it is a reduction product of toluene that can EXAMPLE 6 We have found that toluene can be converted into mixtures that, from the standpoint of energy content, viscosity, cost and ease of manufacture, are outstanding as jet fuels. This is done by a three-stage process wherein (1) toluene is hydroalkylated to a product containing methylcycloalkyl-substituted toluenes, usually in admixture with unconverted toluene, (2) this product mixture is reduced by hydrogenation to form the corresponding methylcycloalkyl-substituted methylcyclohexanes along with methylcyclohexane from toluene reduction, and (3) a part or all of the methylcyclohexane is removed by distillation in order to adjust the jet fuel product to the desired final viscosity and B.t.u. content. Alternatively, most of the unconverted toluene from the first stage can be stripped off for recycle to the hydroalkylation unit. This procedure eliminates the production of excess methylcyclohexane during second stage reduction.

Jet fuel blends with energy contents in the 136,000 B.t.u. per gallon range, including the higher energy polysubstituted products. 139,700 B.t.u./gaL, from the hydroalkylation-reduction stages, with viscosities less than poises at 65 F. and with a substantial cost advantage over existing jet fuels are obtained by this procedure. The process also has the flexibility of providing jet fuel blends with a range of energy-viscosity properties that can be controlled simply by varying the amount of methylcyclohexane removed from the mixture of reduction products.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in carrying out the above method and in the composition set forth without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What We claim is:

1. A method of producing a jet fuel having both a high heat content and an acceptable low temperature viscosity which comprises hydroalkylating benzene, separating from the hydroalkylate a hydrocarbon of the group consisting of dicyclohexylbenzene, tricyclohexylbenzenc and mixtures thereof, and blending said hydrocarbon with about 20% to about 50% of its weight of methylcyclohexane.

2. A jet fuel having both a high heat content and an acceptable low temperature viscosity consisting essentially of a member of the group consisting of dicyclohexylbenzene, tricyclohexylbenzene, dicyclohexylcyclohexane, polycyclohexylcyclohexanes and mixtures thereof together with about 20% to about 50% of their weight of methylcyclohexane.

3. A jet fuel having both a high heat content and an acceptable low temperature viscosity consisting essentially of a mixture of polycyclohexylcyclohexanes together with about 20% to about 50% of their weight of methylcyclohexane.

(References on following page) Appleby et aL: Symposium On Jet Fuels, Preprint of the American Chemical Society, Division of Petroleum References Cited UNITED STATES PATENTS 10/1963 Stahly 260666 P Chemistry, New York, N.Y., vol. 5, No. 4, September 12/1963 Smith et a1. 260-666 P 5 1960011041 to 0-38.

4/1964 Smith et a1. 260-666 P 12/1964 Smith et a1. u 26 66 P HERBERT LEVINE, Primary Examlner 10/1970 Leas 208-15 US. Cl. X.R.

FOREIGN PATENTS 10 (P666 666 PY 6/1960 Great Britain 208-15 

